Air-fuel ratio control system for an automotive engine

ABSTRACT

A first lookup table is provided for storing a plurality of basic fuel injection pulse widths from which one of pulse widths is derived in accordance with engine speed and intake-air pressure. A second lookup table is provided for storing a plurality of maximum correcting quantities for correcting a derived basic fuel injection pulse width in order to correct deviation of air-fuel ratio due to change of valve clearance of the engine. A necessary correcting quantity is obtained by multiplying a learning coefficient and a derived maximum correcting quantity. A desired fuel injection pulse width is obtained by adding the necessary correcting quantity to the derived basic fuel injection pulse width.

BACKGROUND OF THE INVENTION

The present invention relates to an air-fuel ratio control system for an engine of a motor vehicle, and more particularly to a system having an electronic fuel injection system controlled by learning control.

In one type of electronic fuel-injection control, the quantity of fuel to be injected into the engine is determined in accordance with engine operating variables such as mass air flow, intake-air pressure, engine speed and engine load. The quantity of fuel is determined by a fuel injector energization time (injection pulse width).

Generally, a desired injection amount is obtained by correcting a basic quantity of injection with various correction or compensation coefficients of engine operating variables. Basic injection pulse width is derived from a lookup table to provide a stoichiometric air-fuel ratio according to mass air flow or intake-air pressure and engine speed. The basic injection pulse width T_(P) is expressed, for example, as follows.

    T.sub.P =f ( P, N )

where P is intake-air pressure and N is engine speed.

Desired injection pulse width (T) is obtained by correcting the basic injection pulse T_(P) with coefficients for engine operating variables. The following is an example of an equation for computing the actual injection pulse width.

    T=T.sub.P ×K×α×Ka

where K is a set of various coefficients such as coefficients on coolant temperature, full throttle open, etc., α is a feedback correcting coefficient which is obtained from output signal of an O₂ -sensor provided in an exhaust passage, and Ka is a correcting coefficient by learning (hereinafter called learning control coefficient) for compensating the change of characteristics of devices with time in the fuel control system such as, injectors and the O₂ -sensor, due to deterioration thereof. The coefficients K and Ka are stored in lookup tables and drived from the table in accordance with sensed informations.

The control system compares the output signal of the O₂ -sensor with a reference value corresponding to stoichiometric air-fuel ratio and determines the feedback coefficient α so as to converge air-fuel ratio of air-fuel mixture to the stoichiometric air-fuel ratio.

As described above, the basic injection pulse width T_(P) is determined by the intake-air pressure P and engine speed N. However, the intake-air pressure is not always constant, even if the engine speed is the same as previous speed. For example, when a valve clearance (the clearance between an intake (or exhaust) valve-stem tip and a rocker arm) becomes large with time, the valve opening time becomes short. As a result, overlapping times of the intake valve opening time and the exhaust valve opening time become short. Accordingly, quantity of exhaust gas inducted into an intake passage from a combustion chamber during the overlapping time becomes small. Thus, quantity of the intake-air increases. However, the intake-air pressure and hence quantity of fuel injection do not change. Accordingly, the air-fuel ratio becomes large (lean air-fuel mixture). The same result occurs when driving at high altitude.

Such a change of characteristic of a device is also corrected by a learning control coefficient. In a prior art, for example U.S. Pat. No. 4,430,976, a plurality of learning control coefficients are provided with respect to engine operating conditions. Accordingly, a memory having a large capacity is necessary, and construction of the control system and operation become complicated. Further, a long time is consumed for calculating the injection time, which causes delay of the control of air-fuel ratio, and hence aggravations of driveability of a motor vehicle and fuel consumption of the engine.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an air-fuel ratio control system for an automotive engine which may be simplified in construction and operation and may promptly control the air-fuel ratio in response to engine operating conditions, thereby improving driveability of a vehicle.

According to the present invention, there is provided an air-fuel ratio control system for an automotive engine, comprising a first lookup table storing a plurality of basic fuel injection pulse widths from which one of pulse widths is derived in accordance with engine operating conditions, a second lookup table storing a plurality of maximum correcting quantities for correcting a derived basic fuel injection pulse width in order to correct deviation of air-fuel ratio due to change of a characteristic of a device used in the engine, first means for producing a necessary correcting quantity by multiplying a learning coefficient and a maximum correcting quantity derived from the second lookup table, and second means for producing a desired fuel injection pulse width by adding the necessary correcting quantity to the derived basic fuel injection pulse width.

In an aspect of the invention, the engine operating conditions are intake-air pressure and engine speed, and the characteristic of a device is a valve clearance.

The other objects and features of this invention will be apparently understood from the following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a system to which the present invention is applied;

FIG. 2 is a block diagram showing a control system;

FIG. 3 shows graphs showing output voltages of an O₂ -sensor and output voltage of a proportional and integrating circuit (hereinafter called PI circuit);

FIG. 4 is a graph showing relationship between output voltage of the PI circuit and variation ranges of engine speed and intake-air pressure;

FIG. 5 is an illustration showing maps for quantity of fuel injection; and

FIG. 6 is a flowchart showing the operation of the system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an engine has a cylinder 1, a combustion chamber 2, and a spark plug 4 connected to a distributor 3. An engine speed sensor 3a is provided on the distributor 3. An intake passage 5 is communicated with the combustion chamber 2 through an intake valve 7 and an exhaust passage 6 is communicated with the combustion chamber 2 through an exhaust valve 8. In an intake passage 5 of the engine, a throttle chamber 10 is provided downstream of a throttle valve 9 so as to absorb the pulsation of intake-air. A pressure sensor 11 is provided for detecting the pressure of intake-air in the chamber 10 and for producing an intake-air pressure signal. Multiple fuel injectors 12 are provided in the intake passage 5 at adjacent positions of intake valve 7 so as to supply fuel to each cylinder 1 of the engine. An O₂ -sensor 13 and a catalytic converter 14 are provided in the exhaust passage 6. The O₂ -sensor 13 is provided for detecting concentration of oxygen in exhaust gases in the exhaust passage 6.

Output signals from the pressure sensor 11 and the O₂ -sensor 13 are supplied to an electronic control unit (ECU) 15 consisting of a michrocomputer. The engine speed sensor 3a produces an engine speed signal which is fed to the control unit 15. The control unit 15 determines quantity of fuel injected from the injectors 12 and supplies a signal to injectors 12.

Referring to FIG. 2, the electronic control unit 15 comprises a central processor unit (CPU) 16 having an arithmetic and logic circuit (ALU) 17, a read only memory (ROM) 18, and a random access memory (RAM) 19. The ALU 17, ROM 18, and RAM 19 are connected to each other through a bus line 21. An A/D converter 20 is connected to the ALU 17 through a bus line 21a. A sample-hold signal is applied to the A/D converter 20 from the ALU 17. The A/D converter 20 is supplied with analog voltage signals from the pressure sensor 11 and O₂ -sensor 13 to convert the analog voltage signal into a digital signal. An input interface 22 combined with a waveform shaping circuit is supplied with the engine speed signal from engine speed sensor 3a for shaping waveforms of the signal. An output signal of the interface 22 is supplied to ALU 17. A driver 23 produces a pulse signal for driving the injectors 12.

The engine speed signal from the input interface 22 and the intake-air pressure signal from the A/D converter 20 are stored in the RAM 19 through the ALU 17. The air-fuel ratio signal from the A/D converter 20 is compared with a reference voltage signal corresponding to the stoichiometric air-fuel ratio at the CPU 16 at regular intervals. When the air-fuel mixture supplied to the engine is rich compared with the stoichiometric air-fuel ratio, a "1" signal is stored in the RAM 19. When the air-fuel mixture is lean, a "0" signal is stored in the RAM 19. The fuel injection pulse width T is calculated based on the stored data in the RAM 19 and maps 24 and 25 (FIG. 5) stored in the ROM 18 for driving the injectors 12 as described hereinafter. The map 24 is for the basic fuel injection pulse width T_(P) when the valve mechanism has a normal valve clearance. The map 25 stores maximum correcting quantities CLRN for the valve clearance. Each correcting quantity CLRN is a maximum limit value for enriching the mixture. The data T_(P) and CLRN are derived from the maps 24, 25 dependent on the intake-air pressure P and the engine speed N.

Although the maps 24 and 25 are superimposed in FIG. 5 for the convenience of explanation, both maps are provided in individual divisions of ROM 18.

The ALU 17 executes arithmetic processes by reading "1" and "0" data stored in the RAM 19 at regular intervals, as described hereinafter.

As shown in FIG. 3, the air-fuel ratio signal from the O₂ -sensor 13 changes cyclically over the reference valve to rich and lean sides. The ALU 17 produces a feedback correcting signal Fc. When the data changes from "0" to "1", the signal Fc skips in the negative direction (from α1 to α2).

Thereafter, the value of the signal Fc is decremented with a predetermined value at regular intervals. When the data changes from "1" to "0", the signal Fc skips in the positive direction (from α3 to α4), and is incremented with the predetermined value. Thus, the signal Fc has a saw teeth wave as shown in FIG. 3.

In the present invention, the desired fuel injection pulse width T is obtained by adding a necessary correcting quantity NC to the basic injection pulse width Tp. The correcting quantity NC is obtained by multiplying the correcting quantity CLRN by a learning coefficient Kb. Namely the learning coefficient Kb is a rate for obtaining a proper correcting quantity NC from correcting quantity CLRN. The learning coefficient Kb is, for example, 0.5. Thus, the desired fuel injection pulse width T is

    T=(Tp+CLRN×Kb)×α(0≦Kb≦1)

Aforementioned coefficients K and Ka are omitted from the equation. Thus, in the system of the invention, the desired injection pulse width T in the entire operating range according to the intake-air pressure P and engine speed N is obtained by using only two coefficient Kb and α.

Referring to FIG. 6, the operation of the system will be described in more detail.

At starting of the engine at a step S1, a learning coefficient Kb is initially set to "0". The desired fuel injection pulse width T is obtained by calculating the above equation.

When the engine is warmed up and the O₂ -sensor 13 becomes activated, the program proceeds to a step S2 to start a feedback control operation. Average value α8 of the feedback correcting signal Fc from the O₂ -sensor 13 for a period during four times of skipping of signal Fc is obtained as an arithmetical average of maximum values α1, α5 and minimum values α3, α7.

At a step S3, the average value α8 is compared with the stoichiometric air-fuel ratio α0 to obtain a deviation value Δα.

The engine speed is detected at a step S4 whether the engine is in a steady state or not. As shown in FIG. 4, the steady state is decided by ranges Pr and Nr of variations of intake-air pressure and engine speed for a period T of the four times of the skipping. The maximum values and the minimum values of the engine speed N and the intake-air pressure P are obtained. The variation ranges Nr and Pr of the engine speed N and the intake-air pressure P for the period T are obtained from the differences between maximum and minimum values thereof respectively.

If those variation ranges are within set ranges, the engine operation is regarded as being in the steady state, and the program proceeds to a step S5. If those ranges are out of the set ranges, the program returns to the step S3.

At a step S5, it is determined whether the deviation Δα is within a predetermined allowable range (αL≦Δα≦αR) or out of the range. If the deviation Δα is out of the range, the program proceeds to a step S6.

At a step S6, the learning coefficient Kb is rewritten to a value in the range of 0≦Kb≦1 (for example 0.5) such that the deviation Δα becomes within the range (αL≦Δα0≦αR).

If the deviation is within the range, the program returns to the step S3.

Although, in the above described embodiment, the necessary correcting quantity NC is added to the basic injection pulse width T_(P), the following modification may be employed.

Namely, a basic injection pulse width T_(P) and a maximum correcting quantity CLRN are added to produce a maximum injection pulse width Tpmax. A learning coefficient Kc is provided to produce a necessary correcting quantity NCs. The necessary correcting quantity NCs is subtracted from the maximum width Tpmax, therby obtaining a desired pulse width. The calculation expressed as follows.

    T={(T.sub.p +CLRN)-CLRN×Kc}×α

where Kc=1-Kb.

From the foregoing, it will be understood that the air-fuel ratio in the system of the invention is controlled in the entire operating range by using only one learning coefficient, thereby simplifying the construction and operation of the system.

Further, in the system, the lookup table storing maximum correcting quantities serves as a limiter for limiting a maximum quantity of injected fuel, and the lookup table for basic fuel injection pulse width serves as a minimum limiter for the fuel.

While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claims. 

What is claimed is:
 1. An air-fuel ratio control system for an automotive engine, comprising:a first lookup table storing a plurality of basic fuel injection pulse widths from which one of pulse widths is derived in accordance with engine operating conditions; a second lookup table storing a plurality of maximum correcting quantities for correcting a derived basic fuel injection pulse width in order to correct deviation of air-fuel ratio due to change of a characteristic of a device used in the engine; first means for producing a necessary correcting quantity by multiplying a learning coefficient and a maximum correcting quantity derived from the second lookup table; second means for producing a desired fuel injection pulse width in accordance with the necessary correcting quantity and the derived basic fuel injection pulse width.
 2. The system according to claim 1 wherein the engine operating conditions are intake-air pressure and engine speed.
 3. The system according to claim 1 wherein the characteristic of a device is a valve clearance.
 4. The system according to claim 1 wherein the learning coefficient is a value within a range of zero and
 1. 5. The system according to claim 1 wherein the desired fuel injection pulse width is obtained by adding the necessary correcting quantity to the derived basic fuel injection pulse width. 